Synthesis and solvent-free polymerisation of vinyl terephthalate for application as an anode material in organic batteries

Article abstract of DOI:10.1039/C6RA24064J

The synthesis and polymerisation of dimethyl 2-vinylterephthalate M1 for possible applications as an anode material in organic secondary batteries are reported. M1 exhibits a vinyl group as a polymerisable unit while the carboxylate moieties serve as cation (Li⁺, Na⁺) coordinating sites. The gram-scale synthesis of M1 is described via three different routes in order to evaluate the route with the highest overall yield. Furthermore, different conditions for free radical polymerisation are investigated for obtaining polymer P1 with high molecular weights in order to study the impact of immobilising the carboxylate redox-active centres in a polymer on the charge/discharge cycling stability when used in an organic battery. In order to synthesis suitable materials for battery investigations, P1 was post-functionalised to the corresponding lithium salt P1-Li, which was further electrochemically investigated. Cyclic voltammetry measurements showed for P1-Li redox activity in the range of 0.5-1.2 V vs. Li⁺/Li which assigns it as a candidate for the anode. Under the present experimental conditions, the galvanostatic measurements of P1-Li exhibited a specific capacity of 64 mA h g¹. It is further demonstrated that P1-Li shows an improved cycling stability of 83% discharge capacity remaining after 100 cycles compared to the parent monomer (44%).

Full text of DOI:10.1039/C6RA24064J

View Article Online
View Journal
RSC Advances
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: D. Nauroozi, M.
Pejic, P. Schwartz, M. Wachtler and P. Baeuerle, RSC Adv., 2016, DOI: 10.1039/C6RA24064J.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
rsc.li/rsc-advances

Please do not adjust margins
RSC Advances
Page 1 of 9
View Article Online
DOI: 10.1039/C6RA24064J
RSC Advances
ARTICLE
Synthesis and solvent-free polymerisation of vinyl terephthalate for
application as anode material in organic batteries
Djawed Nauroozi,a* Marijana Pejic,b Pierre-Olivier Schwartz,a Mario Wachtler,b and Peter
Received 00th January 20xx,
Accepted 00th January 20xx
Bäuerlea*
.
DOI: 10.1039/x0xx00000x
www.rsc.org/
The synthesis and polymerisation of dimethyl 2-vinylterephthalate M1 for possible applications as anode material in
organic secondary batteries are reported. M1 exhibits a vinyl group as a polymerisable unit while the carboxylate moieties
serve as cation (Li+, Na+) coordinating sites. The gram-scale synthesis of M1 is described via three different routes in order
to evaluate the route with the highest overall yield. Furthermore, different conditions for free radical polymerisation are
investigated for obtaining polymer P1 with high molecular weights in order to study the impact of immobilising the
carboxylate redox-active centres in a polymer on the charge/discharge cycling stability when used in an organic battery. In
order to synthesise suitable materials for battery investigations, P1 was post-functionalised to the corresponding lithium
salt P1-Li, which was further electrochemically investigated. Cyclic voltammetry measurements showed for P1-Li redox
activity in the range of 0.5 – 1.2 V vs Li+/Li which assigns it as a candidate for the anode. Under the present experimental
conditions, the galvanostatic measurements of P1-Li exhibited a specific capacity of 64 mAh g-1. It is further demonstrated
that P1-Li shows an improved cycling stability of 83 % discharge capacity remaining after 100 cycles compared to the
parent monomer (44 %).
dicarboxylates such as terephthalates have been shown to be
suitable candidates for anodes due to their electrochemical
Introduction
reactivity at low potentials.16 However, most of the organic
electrode materials with low molecular weight suffer capacity
fading after a while due to their solubility in the electrolyte. An
intuitive approach to overcome this disadvantage is to use the
corresponding polymers.17,18 Several polymers that show stable
performance over some hundred cycles have already been
investigated as electrode materials.19-21 However, there are just a
few examples of polymers that can be used as anodes.22-24
The concept of using organic molecules as candidates for energy
storage applications has recently attracted increased attention
both for environmental and economic reasons.1-4 Compared to
conventional inorganic electrode materials they offer a potentially
low-cost production due to the low temperature procedures, their
composition of naturally abundant chemical elements (C, H, N, O,
S) that do not involve expensive metals, and their recyclability.
Thus, organic materials represent a promising alternative to their
inorganic counterparts.
Poly(terephthalates) such as poly(ethylene terephthalate),
known as PET, are among the most common polymers, which are
used today. They are produced via condensation reaction of a
carboxylic acid and an alcohol. In order to use poly(terephthalate)
as an electrode active material though, the carboxylate moiety
needs to be accessible for the insertion/de-insertion of metal
cations (Li+ or Na+) during the charge/discharge process in a
battery. We therefore sought the synthesis of a styrene derivative,
2-vinyl-terephthalate M1 (Scheme 1), that allows the
polymerisation of the terephthalate via the vinyl group while the
carboxylate moiety remains accessible for the electrochemical
processes.
The access to an inexhaustible pool of organic molecules
further allows the design of electrode materials suitable for a wide
range of redox potentials. Several classes of organic compounds
such as conjugated polymers, stable organic radicals, organodisul-
fides, and thioethers have already been studied.5-14 Recently,
particular interest is focussed on organic carbonyls as they present
a common functional group and are able to stabilise charges via
delocalisation.3,15 Among carbonyl compounds, conjugated
In this work, we report synthesis of terephthalate-based
monomer M1 as well as the investigation of different conditions for
free radical polymerisation in order to obtain the corresponding
polymers in high molecular weights. The suitability and possible
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 2 of 9
View Article Online
DOI: 10.1039/C6RA24064J
ARTICLE
Journal Name
application of the synthesised polymer as anode material are
further investigated by electrochemical methods such as cyclic
voltammetry and galvanostatic cycling.
Synthesis
Dimethyl 2-vinylterephthalate (M1): In a flame dried three-
necked round bottom flask with a gas inlet tube, 3 g (5.46 mmol) of
5 were placed and dissolved in dry methanol. 1.23 g (10.92 mmol)
potassium tert-butoxide dissolved in dry methanol (10 ml) was
added and the colour of the solution turned to red. After 1 h, in a
separate two-necked round bottom flask para-formaldehyde was
placed and heated to 200 °C which sublimed to formaldehyde gas.
The obtained formaldehyde gas was directed with argon flow to
the first reaction flask through the gas inlet tube. After all para-
formaldehyde was sublimed, the mixture was stirred for 14 h at
room temperature, while the red mixture in the first flask turned to
yellow. The mixture was extracted with dichloromethane, washed
with water and brine. The organic phases were collected, dried
over Na2SO4, and filtered. After removal of the solvent under
reduced pressure a pale yellow solid was obtained, which was
sublimed at a cold finger (75 torr, 80 °C) as a colourless solid in
Experimental
Methods and Instruments:
NMR spectra were recorded on a Bruker AMX 500 (1H NMR: 500
MHz, 13C NMR: 125 MHz) or an Avance 400 spectrometer (1H NMR:
400 MHz, 13C NMR: 100 MHz), normally at 25 °C. Chemical shift
values (δ) are expressed in parts per million using solvent signals
(1H NMR, δH = 7.26 and 13C NMR, δC = 77.0 for CDCl3; 1H NMR,
δH = 2.50 and 13C NMR, δC = 39.52 for DMSO-d6) as internal
standards. Melting points were determined using a Büchi M-565.
Elemental analyses were performed on an Elementar Vario EL (Ulm
University). Cyclic voltammetry experiments were performed with
an Autolab Potentiostat Galvanostat in a three-electrode single-
compartment cell with a platinum working electrode, a platinum
wire counter electrode, and an Ag/AgCl reference electrode.
Ferrocene (Fc) has been added as internal reference. The potentials
have furthermore been recalculated to the Li+/Li reference
electrode using 3.25 V for the oxidation potential of ferrocene vs
Li+/Li. FT-IR spectra were measured with a Perkin Elmer Spectrum
2000. MS/CI experiments were performed using a Finnigan MAT,
SSQ-7000 spectrometer.
1
75 % yield (0.90 g, 4.10 mmol). H-NMR (CDCl3 – 400MHz, ppm): δ
= 8.24 (s, 1H), 7.99-7.86 (m, 2H), 7.41 (dd, 1H, J = 11.2 Hz, 6.4 Hz),
5.76 (d, 1H, J = 17.2 Hz), 5.43 (d, 1H, J = 10.7 Hz), 3.95 (s, 3H), 3.92
(s, 3H); 13C-NMR (CDCl3 – 400MHz, ppm): δ = 167.25, 166.24,
139.53, 134.80, 133.11, 132.31, 130.37, 128.32, 128.10, 117.77,
52.48, 52.43. FT-IR (KBr, 25 °C, cm-1): γ = 2927, 1698, 1565, 1489,
1415, 1303, 1265, 922, 788, 763, 751. MS/CI(+) m/z = 221; mp =
55 °C. Elemental Analysis: C12H12O4 (theoretical); C: 65.45; H: 5.49;
C12H12O4 (found); C: 65.72; H: 6.18.
The battery-related electrochemical investigations were carried
out in three-electrode Swagelok-type cells using lithium metal
(Sigma Aldrich) as the counter and reference electrodes, 3 glass-
fibre sheets (Whatman, GF/A) as separators, and 1 M LiPF6 in a
mixture of ethylene carbonate and dimethyl carbonate (EC:DMC)
1:1 by wt. (UBE Industries) as the electrolyte. The electrodes were
Polymerisation of M1: In a flame dried Schlenk tube with a stir
bar and under argon atmosphere 300 mg (1.36 mmol) of M1 was
placed and the tube was heated to 80 °C while M1 was melted. 0.2
mol % (4 mg) of 2,2’-azobis(2-methylpropionitrile) was added to
the molten M1 and stirring was continued. After 30 min a resin was
obtained and stirring was not possible any more. The temperature
was increased to 150 °C and the tube was maintained at this
temperature for 24 h. After cooling to room temperature the
yellow solid was transferred into 50 ml MeOH. A white precipitate
was formed immediately which was subjected to Soxhlet extraction
with MeOH overnight. The obtained polymer was dried under
vacuum and gave the polymer P1 in 80 % (240 mg) as a colourless
solid. 1H-NMR (CDCl3 – 400MHz, ppm): δ = 7.91-7.00 (3H), 4.01-
3.51 (3H), 3.50-2.78 (3H), 1.98-1.00 (3H); FT-IR (KBr, 25 °C, cm-1):
γ = 2953, 2916, 2848, 1723, 1435, 1286, 1240, 1192, 1116, 753.
Synthesis of P1-COOH: 300 mg P1 were dissolved in a mixture
of 15 ml THF/MeOH (2:1) and 220 mg (4 eq.) NaOH was added to
the solution. The mixture was heated to reflux for 14 h. A
precipitate was formed which was collected and washed
excessively with MeOH. The product was obtained after drying
under vacuum as a colourless powder in 80 % yield (210 mg). FT-IR
(KBr, 25 °C, cm-1): γ = 2200-3500, 1702, 1568, 1495, 1419, 1233,
1125, 1054, 916, 880, 853, 757.
prepared via
a solvent-free preparation method using poly-
tetrafluoroethylene (PTFE, Dyneon) as the binder. The active
material (56.6 wt. %) and carbon black (SuperP-Li, Timcal; 33 wt. %)
were homogenised by ball-milling (400 rpm, 10 min) before PTFE
powder (10.4 wt. %) was added in a mortar to form a smooth
paste. After calendering, composite electrode discs with diameters
of 12 mm and a typical thickness of 170 – 190 µm were cut out of
the film, pressed onto aluminium expanded metal mesh (3 t;
1 min), then dried overnight under dynamic vacuum at 80 °C, and
finally stored under dry Ar. The cells were assembled in an Ar-filled
glove box. Cyclic voltammetry was performed with a VMP3
electrochemical workstation from Biologic. The experiments were
run with a scan rate of 0.1 mV s-1 and a potential range of 0.5 –
2.0 V vs Li+/Li. Galvanostatic cycling tests were performed with a
CTS battery cycler from Basytec. The cells were charged and
discharged with a constant current of 0.1 C for 100 cycles, within
0.5 – 2.0 V vs Li+/Li. The C-rates were calculated based on the
theoretical capacity of the corresponding active material (the-
oretical capacity of P1-Li = 263 mAh g-1, 1C rate = 263 mA g-1, C/3
rate = 87.7 mA g-1). All electrochemical measurements were carried
out at room temperature and repeated in order to guarantee
reproducibility. The specific currents and capacities are referred to
the mass of active material.
Synthesis of P1-Li using LiHMDS: 200 mg (1.0 mmol) P1-COOH
were dissolved in 20 ml dry THF and 700 mg (4.2 mmol) lithium
hexamethyldisilazide (LiHMDS) was added in portions. Then the
mixture was heated to 60 °C for 14 h. The formed precipitate was
collected and washed with hot THF. The product was obtained after
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 3 of 9
View Article Online
DOI: 10.1039/C6RA24064J
Journal Name
ARTICLE
drying under vacuum as a pale yellow powder in 70 % yield (150
resulting in a self-discharge of the cell. An intuitive approach to
circumvent this problem is the transformation of the electrode-
active material into a polymer. With the use of M1 we intended to
prepare stable polymers P1 that show low solubility in the
electrolyte mixtures of a battery (Scheme 1, last step). Hence,
different conditions for a free radical polymerisation reaction of
monomer M1 with 2,2´-azobis(2-methylpropionitrile) (AIBN) as
radical starter have been investigated. All polymerisation
conditions as well as the corresponding characterisation data for
the obtained polymers are summarised in Table 1. The GPC
chromatograms are shown in Figure 1. First attempts to polymerise
M1 in THF as solvent at 60 °C gave polymers with molecular
weights of around Mn = 18 kDa corresponding to approximately 90
repeating units (Figure 1, black curve). In order to obtain polymers
with higher molecular weights, we tried to improve the
polymerisation process by changing the solvent from THF to
dioxane or N-methylpyrrolidone (NMP), which allow performing
the reaction at higher temperatures, 100 °C and 150 °C,
respectively. However, both solvents gave polymers with molecular
weights of approximately 15 kDa (Figure 1, orange and turquoise
curves). The same trend was observed when different radical
starters such as dibenzoyl peroxide (DBPO) or the water soluble
2,2’-azobis(2-methylpropionamidine) dihydrochloride were used
(data and chromatograms not shown).
mg). FT-IR (KBr, 25 °C, cm-1): γ = 1517, 1428, 864, 500.
Results and discussion
Synthesis of the monomer
The synthesis of monomer M1 is shown in Scheme 1. Although
molecules 1-5 as well as M1 are literature-known (s. ESI), for some
of them a synthesis procedure and/or proper characterisation are
missing, which is described here. In the first step, the commercially
available 2-methylterephthalonitrile 1 was treated with NaOH in
presence of diethylene glycol (DEG) to obtain 2-methylterephthalic
acid 2 in quantitative yield. Subsequent esterification under acidic
conditions gave methyl ester 3 quantitatively. Bromination of the
methyl group using N-bromosuccinimide (NBS) and subsequent
transformation to the corresponding phosphonium salt provided 5
that serves as a precursor for the following Wittig reaction.25 Un-
fortunately, a convenient synthesis of monomer M1 with commer-
cially available formaldehyde is not possible due to the presence of
water in the methanol solutions of formaldehyde. To circumvent
this obstacle, formaldehyde was generated in situ by decomposi-
tion of the para-formaldehyde polymer into the corresponding
monomers at 200 °C. Thus, the formaldehyde was obtained as a
gas, which could be easily directed to the reaction flask, where the
phosphonium salt was already treated with potassium tert-
butylate. This reaction gave the desired 2-vinylterephthalate M1
after 14 h reaction time at room temperature. Purification via flash
chromatography and recrystallisation from ethyl acetate gave the
monomer M1 in 75 % yield. Although the overall yield of this multi-
step synthesis is satisfying (65 % over 5 steps), other routes have
also been investigated in order to reduce the number of synthetic
steps and to gain better yields (details in Supporting Information).
However, the overall yields of the two other routes were less than
the one shown here.
Benefiting from the low melting point of M1 (mp = 55 °C), a
solventless free radical polymerisation was tested. For this
purpose, M1 was melted in a dried tube and the melt was stirred
continuously. Then, the radical starter was added and the stirring
continued for 8 h. In this fashion, we obtained in a first attempt a
polymer that showed
a bimodal distribution in the GPC
chromatogram with molecular weights of 400 kDa and 25 kDa,
respectively (Figure 1, green curve). The promising formation of
polymers with longer chains using this method prompted us to test
other conditions in order to obtain monomodal polymers with high
Polymerisation experiments
The solubility of organic molecules in electrolytes is among the
most serious obstacles for their use in batteries. On the one hand,
the electrode capacity decreases as it dissolves, on the other hand,
the dissolved redox-active molecules can act as redox shuttles
Scheme 1: Synthesis of vinyl terephthalate monomer M1. For the
polymerisation conditions towards P1, see text and Table1.
Figure 1: GPC chromatograms of polymerisation experiments of
M1.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 4 of 9
View Article Online
DOI: 10.1039/C6RA24064J
ARTICLE
Journal Name
Table 1: Polymerisation conditions of M1 and characterisation data of the corresponding polymers. 2,2’-Azobis(2-methylpropionitrile) was
used as radical starter in all cases. PDI = Polydispersity index; DP = Degree of polymerisation.
Concentration
Entry
Solvent
T [°C]
Time [h]
Mn [kDa]
PDI
DP
[mol/L]
1
2
3
4
Dioxane
NMP
THF
0.45
0.45
0.45
bulk
100
150
60
24
24
24
8
15
16
1.5
1.5
1.6
1.3
4.6
5.6
2.6
70
70
18
90
-
80
400
25
1800
115
900
2500
5
6
-
-
bulk
bulk
80
24
24
200
550
80-150
1
molecular weights. In another attempt, simple elongation of the
reaction time led to a polymer with a monomodal distribution in
the GPC chromatogram with approx. 200 kDa (blue curve).
missing in the H NMR spectrum of the polymer P1 indicating the
reaction of the vinylic group (Figure 2, bottom). In addition, a broad
signal at 1.80-1.25 ppm is observed that corresponds to the
protons of the aliphatic chain. It is also interesting to notice that
the methyl ester signals are much more separated in the polymer
NMR (3.82 ppm and 3.24 ppm) compared to the monomer,
supposedly because of the closer vicinity of the alkyl chain to the
carboxylate groups. At the same time, the aromatic signals are
merged into each other and give a broad signal centred at 7.46
ppm.
Along with the variation of reaction times a gradual increase of
the temperature from 80 °C to 150 °C during the polymerisation
reaction was tested as well. Much to our satisfaction, polymers
with molecular weights of around 550 kDa and a polydispersity
Index (PDI) of 2.6 were obtained (Figure 1, pink curve). Triggered by
the obtained results, reaction times of longer than 24 h were also
investigated; however, no significant improvement in obtaining
longer chains was observed. It is noteworthy that upon repeating
the synthesis of P1, both the solventless as well as the solution-
based polymerisations gave reproducible results.
Post-functionalisation of polymer P1
For the synthesis of the Li-salt P1-Li that would serve as battery
material, the esters of the polymer P1 were first hydrolysed to the
corresponding carboxylic acids. After deprotonation, Li+-ions were
inserted via two synthetic routes (Scheme 2). The first route
(Scheme 2, Route A) follows a literature-known procedure,16 where
the hydrolysis of the ester groups and the insertion of the Li+-ions
occur in one step by using LiOH. For this purpose, the polymer was
dissolved in THF and LiOH was added to the solution. After 12 h
reaction time, a precipitate was observed indicating the formation
of the polymer Li-salt P1-Li. As the product is well soluble in water,
but shows low solubility in THF, the water phase was dropped into
an excess amount of THF. The obtained precipitate was subjected
to centrifugation and collected by decantation. Since LiOH is
soluble in a certain amount of water as well, this procedure was
repeated several times in order to obtain the pure polymer P1-Li.
To ensure the purity of the material and to synthesise the Li-
salt in a more convenient way, another route was sought (Scheme
2, Route B). Following a two-step synthesis, the ester polymer P1
was first hydrolysed with NaOH in a THF/MeOH mixture. Under
these conditions, the desired polymer with two carboxylic acid
groups P1-COOH precipitated from the solution. The pure
compound was obtained after an excessive wash of the precipitate
with MeOH in a yield of 80 %. Treatment of P1-COOH with LiHMDS
in dry THF at 50 °C overnight led to the formation of the target P1-
Li. The obtained material was washed excessively with hot THF in
order to get rid of the formed by-products.
The obtained polymers were characterised by 1H NMR
spectroscopy. Figure 2 shows both the 1H NMR spectra of the
monomer M1 and the corresponding polymer P1. The spectrum of
the monomer M1 (top) displays the aromatic signals of the protons
of the phenyl ring at 7.93 ppm and 8.24 ppm, respectively. In
addition to the signals for the methyl ester groups at 3.94 ppm and
3.92 ppm, two doublets can be observed at 5.43 ppm and 5.76
ppm, respectively, as well as a quartet at 7.41 ppm that correspond
to the vinylic protons. These proton signals are undoubtedly
Figure 2: 1H NMR spectra (400 MHz, CDCl3) of the monomer M1
(top) and the corresponding polymer P1 (bottom).
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 5 of 9
Journal Name
View Article Online
DOI: 10.1039/C6RA24064J
ARTICLE
Scheme 2: Synthesis of the polymer Li-salt P1-Li via two different
routes.
To validate the insertion of the Li+-ions, the two polymers P1
and P1-Li were characterised by IR-spectroscopy (Figure 3). The
polymer P1 showed two characteristic bands at 1723 cm-1 and
1286 cm-1 that correspond to the stretching vibration of the
carbonyl (C=O) and the alkyl grou ps of the polymer backbone (C-
C), respectively. These two bands are markedly shifted in case of
the polymer lithium salt P1-Li. The carbonyl band is shifted by
about 200 cm-1 to lower wavenumbers and can be observed at
1517 cm-1, while the band for the C–C-bond vibrations are shifted
by about 150 cm-1 to higher wavenumbers and can be observed at
1428 cm-1. These significant shifts indicate the change of the
vibrational character in the two materials. Moreover, a significant
band can be additionally observed at approximately 500 cm-1 in the
IR spectrum of P1-Li corresponding to the Li-O deformation
vibration, which confirms the successful substitution reaction of
the Li+-ions. This band is clearly missing in the ester-functionalised
polymer P1.
Figure 3: IR-spectroscopic analyses of the ester polymer P1 (top)
and the Li-salt polymer P1-Li (bottom).
phosphate (TBAPF6) as electrolyte. The cyclic voltammogram of the
monomer M1 (Figure 4, left) showed a reversible reduction wave
at -2.35 V vs Fc+/Fc (~ 0.9 V vs Li+/Li) with two electrons involved
referring to the reduction of the methyl esters. In case of the
polymer P1 (Figure 4, right), the cyclic voltammogram showed a
reduction process at -2.53 V vs Fc+/Fc (~0.72 V vs Li+/Li) which is
about 300 mV cathodically shifted compared to the monomer. The
observed redox process showed a broad wave for the reduction
which is common for polymers. The re-oxidation of the same
polymer showed an impetuous drop of current at -2.49 V vs Fc+/Fc
that recovers again and continues giving a broad wave. Assuming
that the polymer exists as a coil, this observation may be related to
the faster re-oxidation of the outer units of the polymer due to
their easier accessibility while that of the units inside the coil needs
more time because of the diffusion dependency.
Electrochemical characterisation of M1 and P1 in solution
Monomer M1 and the corresponding polymer P1 were characte-
rised electrochemically by means of cyclic voltammetry (CV). The
measurements were performed in
a 0.1 M solution of the
compound in THF with n-tetrabutylammonium hexafluoro-
Figure 4: Cyclic voltammograms of monomer M1 (left) and the corresponding polymer P1 (right). Measurements were performed in 0.1 M
solutions of the compounds in THF with TBAPF6 as electrolyte; scan rate = 200 mV s-1; working electrode area: 0.79 mm2; referenced against
Fc+/Fc (*).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 6 of 9
View Article Online
DOI: 10.1039/C6RA24064J
ARTICLE
Journal Name
Figure 5: Electrochemical results of P1-Li in 1 M LiPF6 / EC:DMC (1:1 by wt.) of all electrodes containing P1-Li (56.6 wt. %), PTFE (10.4 wt. %)
and carbon black (33 wt. %). (a) CV (0.1 mV s-1, 5 cycles). (b) Galvanostatic charge-discharge cycling (C/3) of P1-Li (blue) compared to
dilithium terephthalate (red). Capacity plots (charge capacity: closed symbols; discharge capacity: open symbols) and potential profiles of
cycle 1, 2, 10, 50, 100, 150 and 200 (inset).
Measurements on polymers with different molecular weights of 18
kDa and 200 kDa led to the same observation (see SI, Figure 2). A
similar electrochemical investigation of the Li-salt P1-Li was not
possible because of the insolubility of the material in any common
organic solvent. Nevertheless, considering the structural similarity
of the Li-salt and the methyl ester derivative, the reduction of M1
as well as of P1 at low potentials underlines the principal suitability
of this class of compounds as anode material in batteries. It is
worth emphasising that the polymer salt P1-Li is completely in-
soluble in any organic solvent. In particular, the insolubility in
common battery electrolytes such as ethylene carbonate/dimethyl
carbonate underlines the superiority of polymeric materials in
terms of cycling stability in comparison with the corresponding
monomers, which is discussed below.
The galvanostatic charge-discharge profile of P1-Li shown in
Figure 5b (blue curve) confirms the high irreversible loss in the 1st
cycle. One reason is the high amount of carbon black (33 wt.%) in
the electrode, which is indispensable for sufficient conductivity
within the electrode, because this kind of active materials is usually
non-conductive. It is known that carbon black itself exhibits a large
irreversible capacity below 1 V vs Li+/Li caused by its high surface
area and functionalities, which offer many possibilities for
reductive side reactions.28,29 These side reactions are visualised in
the CV (Figure 5a, black curve) by the increased reductive currents
during the 1st cycle.
The reversible capacity amounts to 58 mAh g-1 (at the C/3 rate
used for the galvanostatic cycling test), which is 22% of the theo-
retical capacity of P1-Li (263 mAh g-1) (Figure 5b, blue curve).
Apparently not all of the material takes part in the electrochemical
reaction. A possible reason could be an insufficient electronic
conductivity within the electrode. As P1-Li is an electronic insulator,
only those areas of the material will electrochemically react which
are in contact with the carbon black. Especially for large particles
the reaction may therefore be limited to the particle surface. We
expect the reversible capacity to increase by decreasing the particle
size and by optimising the distribution of and contact between the
carbon black and the active material. Generally, literature-known
polymers which are used as cathode materials often exhibit lower
specific capacities than the calculated theoretical capacities of their
particular units.30,31
Electrochemical investigations of P1-Li as anode material for
organic batteries
The CV of the polymeric anode material P1-Li shows a redox range
of 0.5 – 1.2 V vs Li+/Li (Figure 5a) (consistent with the results of P1
in Figure 4). Only few organic compounds have been described
which work at such low potentials upon insertion/de-insertion
mechanism, and almost all of them are of low molecular
weight.16,26 One example of a polymeric compound is diethyl
terephthalate-functionalised polythiophenes, which have been
reported recently27- unfortunately no specific capacities are given.
P1-Li is another polymer active in this potential region. Generally, a
low anode potential is the key to obtaining large voltages and high
energies of the full cell.
To test the effect of immobilising the redox-active unit in a
polymer, we compared the electrochemical performance of P1-Li
to that of the related literature-known monomeric dilithium
terephthalate. Dilithium terephthalate was synthesised as
previously described,16 and processed into electrodes and tested
under the same preparation and cycling conditions as P1-Li. The CV
measurements (see SI, Figure 3) indicate higher capacities than for
the polymer P1-Li, and a significant irreversible capacity. This is
On repeated cycling there is a major change taking place
between the first and the second CV cycle, whereas from the
second cycle onwards the CV curves change only little. Obviously,
some kind of formation process takes place during the first charge,
giving rise to significant irreversible capacity.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 7 of 9
View Article Online
DOI: 10.1039/C6RA24064J
Journal Name
ARTICLE
reflected in the galvanostatic measurements (Figure 5b). Figure 5b
shows, furthermore, that although the polymer (blue) provides a
lower initial specific capacity than dilithium terephthalate (red), its
cycling stability is significantly improved, resulting in much slower
capacity decay. The ratio of the discharge capacities of the 100th
cycle to that of the 1st cycle is 85% for P1-Li and only 40% for
dilithium terephthalate. The inferior cycling stability of the low-
molecular-weight terephthalate is presumably associated with the
dissolution of the active material into the electrolyte.16 As well
swelling of the material / electrode due to uptake of solvent from
the electrolyte may contribute to the capacity fade via loss of
electronic contact within the electrode material. In the case of P1-
Li the corresponding active units are incorporated into a polymer
backbone preventing their dissolution. Thus, regarding long term
cycling the polymer P1-Li seems to be the more suitable candidate
for battery applications. Last but not least, the polymer P1-Li shows
a better rate capability than monomeric dilithium terephthalate
(see SI, Figure 4).
This class of polymers seems to be promising for application as
anode material in battery applications. Nonetheless, further
investigations on how to reduce the irreversible capacity (especially
in the first cycle) and on how to increase the practical reversible
capacity are needed before it can be used in commercial cells.
Acknowledgements
The authors are indebted to the German Federal Ministry of
Education and Research (BMBF) for financial support within the
project “LiEcoSafe” (contract no. 03X4636A/B). Dr Günther Götz
and Dr Elena Mena-Osteritz (Ulm University) are sincerely
acknowledged for fruitful discussions, and Dagmar Weirather-
Köstner (ZSW) for complementary electrochemical measurements.
The authors thank Zeynab Nikpoor for the graphical illustration of
the TOC Figure.
References
1 Y. Liang, Z. Tao, J. Chen, Adv. Energy Mater., 2012, 2, 742.
2 Z. P. Song, H. S. Zhou, Energy Environ. Sci., 2013, 6, 2280.
3 P. Poizot, F. Dolhem, Energy Environ. Sci., 2011, 4, 2003.
4 M. Armand, J. M. Tarascon, Nature, 2008, 451, 652.
5 K. Sakaushi, E. Hosono, G. Nickerl, T. Gemming, H. Zhou, S. Kaskel,
J. Eckert, Nat. Commun., 2013, 4, 1485.
Conclusions
The monomer dimethyl 2-vinylterephthalate M1, which was
synthesised in this work, exhibits a vinyl group as a polymerisable
unit while the carboxylate moieties serve as cation coordinating
sites. Synthesis of M1 was performed via three different routes in
order to evaluate the route with the highest overall yield. However,
in our hands the route with the highest number of synthetic steps
gave the highest overall yield. Different conditions were evaluated
to obtain polymers of M1 via free radical polymerisation. Solution-
based polymerisation with different solvents gave polymers with
merely low molecular weights of 18 kDa in average. Taking
advantage of the low melting point of M1 it was possible to
perform the polymerisation reaction in the bulk. Under these
conditions polymer P1 with high molecular weights of 550 kDa
could be obtained with reaction times of 24 h and reaction
temperatures of approx. 150 °C. With the aim to use polymers as
electroactive materials in organic batteries, P1 was post-
functionalised to the corresponding lithium salt P1-Li via two
different routes. The route with stepwise hydrolysis and Li-insertion
using LiHMDS was more convenient in terms of better yields and
sophisticated purification.
6 L. Zhan, Z. Song, J. Zhang, J. Tang, H. Zhan, Y. Zhou, C. Zhan,
Electrochim. Acta, 2008, 53, 8319.
7 L. Zhan, Z. Song, N. Shan, J. Zhang, J. Tang, H. Zhan, Y. Zhou, Z. Li,
C. Zhan, J. Power Sources, 2009, 193, 859.
8 N. Oyama, T. Tatsuma, T. Sato, T. Sotomura, Nature, 1995, 373,
598.
9 S. R. Deng, L.-B. Kong, G.-Q. Hu, T. Wu, D. Li, Y.-H. Zhou, Z.-Y. Li,
Electrochim. Acta, 2006, 51, 2589.
10 T. Janoschka, M. D. Hager, U. S. Schubert, Adv. Mater., 2012, 24,
6397.
11 T. Jähnert, M. D. Hager, U. S. Schubert, J. Mat. Chem. A, 2014, 2,
15234.
12 K. Oyaizu, H. Nishide, Adv. Mater., 2009, 21, 2339.
13 K. Nakahara, K. Oyaizu, H. Nishide, Chem. Lett., 2011, 40, 222.
14 P. Novák, K. Müller, K. S. V. Santhanam, O. Haas, Chem. Rev.,
1997, 97, 207.
15 B. Häupler, A. Wild, U. S. Schubert, Adv. Energy Mater., 2015, 5,
P1-Li was investigated electrochemically using cyclic
voltammetry and galvanostatic measurements. The CVs of P1-Li
showed redox activity in a potential range of 0.5 – 1.2 V vs Li+/Li
which assigns the material as a candidate for anode. P1-Li showed
high irreversible losses in the first cycle, which is symptomatic for
most organic materials. Galvanostatic charge-discharge cycling
reveals a specific capacity of 58 mAh g-1 for P1-Li (at a C/3 rate)
which is only 22 % of the theoretical capacity. However, the
polymer displays a good cycling stability, still showing a discharge
capacity of 85% after 100 cycles. Especially the comparison with
the parent monomer dilithium terephthalate with only 40%
discharge capacity after 100 cycles reveals the superiority of the
polymer in terms of cycling stability.
1402034.
16 M. Armand, S. Grugeon, H. Vezin, S. Laruelle, P. Ribiére, P. Poizot,
J.-M. Tarascon, Nature Mater., 2009, 8, 120.
17 L. M. Zhu, W. Shi, R. R. Zhao, Y. L. Cao, P. X. Ai, A. W. Lei, H. X.
Yang, J. Electroanal. Chem., 2013, 688, 118.
18 J. Heinze, B. A. Frontana-Uribe, S. Ludwigs, Chem. Rev., 2010,
110, 4724.
19 Z. Song, Y. Qian, X. Liu, T. Zhang, Y. Zhu, H. Yu, M. Otani, H. Zhou,
Energy Environ. Sci., 2014, 7, 4077.
20 Z. Song, Y. Qian, T. Zhang, M. Otani, H. Zhou, Adv. Sci., 2015, 2,
1500124.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 8 of 9
View Article Online
DOI: 10.1039/C6RA24064J
ARTICLE
Journal Name
21 Z. Song, Y. Qian, M. L. Gordin, D. Tang, T. Xu, M. Otani, H. Zhan,
H. Zhou, D. Wang, Angew. Chem. Int. Ed., 2015, 54, 13947.
22 L. Chen, W. Li, Y. Wang, Y. Xia, RSC Adv., 2014, 4, 25369.
23 W. W. Deng, S. M. Liang, X. Y. Wu, J. F. Qian, Y. L. Cao, X. P. Ai, J.
W. Feng, H. X. Yang, Sci. Rep., 2013, 3, 2671.
24 H. Qin, Z. P. Song, H. Zhan, Y. H. Zhou, J. Power Sources, 2014,
249, 367.
25 G. Wittig, U. Schöllkopf, Chem. Ber., 1954, 87, 1318.
26 a) W. Walker, S. Grugeon, H. Vezin, S. Laruelle, M. Armand, F.
Wudl, J.-M. Tarascon, J. Mater. Chem., 2011, 21, 1615. b) T. Yasuda,
N. Ogihara, Chem. Commun., 2014, 50, 11565.
27 L. Yang, X. Huang, A. Gogoll, M. Strømme, M. Sjödin, Electrochim.
Acta, 2016, 204, 270.
28 L. Fransson, T. Eriksson, K. Edström, T. Gustafsson, J. O. Thomas,
J. Power Sources, 2001, 101, 1.
29 K. Takei, N. Terada, K. Kumai, T. Iwahori, T. Uwai, T. Miura, J.
Power Sources, 1995, 55, 191.
30 J. Xiang, C. Chang, M. Li, S. Wu, L. Yuan, J. Sun, Cryst. Growth
Des., 2008, 8, 280.
31 T. L. Gall, K. H. Reiman, M. C. Grossel, J. R. Owen, J. Power
Sources, 2003, 119–121, 316.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 9
RSC Advances
View Article Online
DOI: 10.1039/C6RA24064J
Solvent-free polymerisation of vinyl terephthalate was used to obtain high molecular weight
polymers, whose corresponding Li-salts underlined the superiority of polymers regarding long
term stability in comparison with the monomeric counterparts.